1. Field of the Invention
The present invention relates generally to a cellular wireless communication system, and more particularly, to a method and apparatus for transmitting/receiving pilot signals.
2. Description of the Related Art
In cellular wireless communication systems, for demodulation of received data and control information, synchronization and cell search should first be achieved between a transmitter and a receiver. The downlink synchronization and cell search process refers to a process of determining a frame start point of physical channels transmitted in a cell to which a User Equipment (UE) belongs, and detecting a cell-specific scrambling code applied during transmission of the physical channels. This process is referred to herein as a ‘cell search process’. The cell search process is achieved by detecting a code of a downlink Synchronization Channel (SCH) by a UE.
FIG. 1 illustrates an Orthogonal Frequency Division Multiplexing (OFDM)-based downlink frame structure and a transmission point of a synchronization channel for Enhanced Universal Terrestrial Radio Access (EUTRA) which is the next generation mobile communication standard of the 3rd Generation Partnership Project (3GPP).
As illustrated in FIG. 1, a 10-ms radio frame 100 is composed of 10 subframes, and each subframe 106 is composed of two slots 101, 102. Generally, seven OFDM symbols 105 are transmitted in one slot. In the downlink, a Primary Synchronization Channel (P-SCH) 104 and a Secondary Synchronization Channel (S-SCH) 103 are transmitted in slots 101, 102 defined in each subframe 106.
In the EUTRA system, a UE acquires slot timing synchronization from the P-SCH in a first step of cell search. The slot timing synchronization acquisition process is achieved by calculating, by a UE, a correlation between a scrambling code applied to the P-SCH (P-SCH Scrambling Code; hereinafter referred to as ‘PSC’) and a received signal, and searching for a time where a high correlation is generated. There are three PSC codes used for P-SCH, and one cell transmits the P-SCH using one of the three PSC codes.
In a second step, the UE checks, from the S-SCH, frame timing synchronization and a cell code group including a cell-specific scrambling code applied to the corresponding cell. This is achieved by detecting a scrambling code applied to the S-SCH (S-SCH Scrambling Code; hereinafter referred to as SSC). In this case, as shown in FIG. 1, P-SCH 104 and S-SCH 103 are transmitted through adjacent OFDM symbols in one slot. Therefore, the UE performs synchronization detection (or coherent detection) for removing an influence of channels on S-SCH signals, by using the P-SCH detected in the first step of cell search as a channel estimation pilot for S-SCH detection. The interference-removed S-SCH signals are then detected, making it possible to improve S-SCH detection performance. There are 170 SSC codes used for the S-SCH, and one cell transmits the S-SCH using one of the 170 SSC codes.
One cell transmits the P-SCH using one of the three PSCs, and transmits the S-SCH using one of the 170 SSCs. Therefore, it is possible to find a cell Identifier (ID) with a combination (or set) of the PSC and SSC codes. That is, a UE selects one of 170 cell group IDs depending on a received S-SCH transmitted by the corresponding cell, and selects a cell ID belonging to the cell group ID from the P-SCH received in advance of the S-SCH, thereby determining which cell ID among a total of 510 cell IDs the Node B has. The cell selects one of the 510 cell IDs, and scrambles a downlink pilot channel (or reference signal) using one predetermined cell scrambling code mapped to the selected ID. Therefore, in the final step, the UE selects one scrambling code mapped to the decided cell ID, and descrambles a received pilot channel with it to determine whether the pilot signal has been normally received. That is, based on whether the pilot signal has been normally received or not, the UE determines whether the decision on a cell ID has been made correctly through the cell search process.
The cell search process of a UE has been described above. The number of cell IDs that the UE can select in the cell search process is limited to 510. While the number of Node Bs from which one UE can receive cell IDs is limited, the 510 cell IDs are enough for one UE. However, Home Node B has recently been considered a kind of the cell. The Home Node B, a small-sized Node B, can be arbitrarily installed in a place desired by a user. Therefore, in the possible situation where the 510 cell IDs are not enough due to the use of Home Node Bs, the UE may have difficulty in distinguishing Node Bs, causing communication problems.